The present invention relates to an optical compensatory sheet comprising an optically uniaxial or optically biaxial transparent stretched film. The invention also relates to an optical compensatory sheet comprising the transparent stretched film and an optically anisotropic layer formed from liquid crystal molecules. The invention further relates to an ellipsoidal polarizing plate and a liquid crystal display equipped with the optical compensatory sheet.
A liquid crystal display generally has a liquid crystal cell, polarizing plates and an optical compensatory sheet (phase retarder). In a display of transmission type, two polarizing plates are placed on both sides of the liquid crystal cell, and one or two optical compensatory sheets are placed between the liquid crystal cell and the polarizing plate. On the other hand, a display of reflection type comprises a reflection plate, a liquid crystal cell, one optical compensatory sheet and one polarizing plate piled up in this order.
The liquid crystal cell comprises a pair of substrates, rod-like liquid crystal molecules and an electrode layer. The rod-like liquid crystal molecules are provided between the substrates, and the electrode layer has a function of applying a voltage to the rod-like liquid crystal molecules. According to alignment of the rod-like liquid crystal molecules in the cell, various display modes are proposed. Examples of the display modes for transmission type include TN (twisted nematic) mode, IPS (in-plane switching) mode, FLC (ferroelectric liquid crystal) mode, OCB (optically compensatory bend) mode, STN (super twisted nematic) mode, VA (vertically aligned) mode and ECB (electrically controlled birefringence) mode. Examples of the modes for reflection type include TN mode, HAN (hybrid aligned nematic) mode and GH (guest-host) mode.
The optical compensatory sheet is widely used in various liquid crystal displays because it prevents the displayed image from undesirable coloring and enlarges a viewing angle of a liquid crystal cell. As the optical compensatory sheet, a stretched birefringent polymer film has been conventionally used.
In place of the stretched birefringent polymer film, an optical compensatory sheet having an optically anisotropic layer formed from liquid crystal molecules (such as discotic liquid crystal molecules, rod-like liquid crystal molecules) has been proposed. The optically anisotropic layer is provided on a transparent film. Since the liquid crystal molecules have various alignment forms, an optical compensatory sheet obtained from the liquid crystal molecules has a specific optical characteristic that cannot be obtained from the conventional stretched birefringent polymer film.
The optical characteristic of the optical compensatory sheet is designed according to that of the liquid crystal cells namely, according to display mode of the liquid crystal cell. If the optical compensatory sheet is made with liquid crystal molecules, various optical characteristics can be realized according to the display mode of the liquid crystal cell.
For liquid crystal cells of various display modes, optical compensatory sheets using liquid crystal molecules have been proposed. For example, an optical compensatory sheet for liquid crystal cell of TN mode is described in Japanese Patent Provisional Publication No. 6(1994)-214116, U.S. Pat. Nos. 5,583,679, 5,646,703 and German Patent Publication No. 3,911,620A1. An optical compensatory sheet for liquid crystal cell of IPS or FLC mode is described in Japanese Patent Provisional Publication No. 10(1998)-54982. An optical compensatory sheet for OCB or HAN mode is described in U.S. Pat. No. 5,805,253 and International Patent Application No. WO96/37804. An optical compensatory sheet for STN mode is described in Japanese Patent Provisional Publication No. 9(1997)-26572. An optical compensatory sheet for VA mode is described in Japanese Patent No. 2,866,372.
If liquid crystal molecules are used in place of a conventional stretched birefringent polymer film, a liquid crystal cell can be optically compensated more correctly than before. For example, a liquid crystal cell comprising many essentially vertically aligned rod-like liquid crystal molecules (e.g. liquid crystal cell of VA, OCB or HAN mode) can be effectively compensated by an optical compensatory sheet in which the discotic liquid crystal molecules are aligned under the condition that an average inclined angle between the discotic plane of the molecule and the surface of the transparent film is less than 5xc2x0.
However, it is still very difficult to completely compensate a liquid crystal cell by the liquid crystal molecules alone, and hence it has been wanted to further improve the viewing angle character of liquid crystal display.
International Patent Application No. WO00/49430 discloses a liquid crystal display having a liquid crystal cell strictly compensated for improving the viewing angle character. In this display, the cell is compensated by an optical compensatory sheet comprising an optically anisotropic transparent film and a thereon-provided optically anisotropic layer formed from discotic liquid crystal molecules.
An optical compensatory sheet comprising a transparent stretched birefringent film can improve quality of an image on a liquid crystal display at a certain degree. An optical compensatory sheet comprising a transparent stretched polymer film and an optically anisotropic layer formed from liquid crystal molecules can also improve quality of an image. However, the viewing angle still changes according to the viewing position on the displaying screen of the liquid crystal display. Further, troubles of light leaks are sometimes observed in the image. The image may leak light along the frame of the image where the image should be black. The image may also leak light as a light spot within a black area.
An object of the present invention is to provide an optical compensatory sheet that uniformly compensates a liquid crystal cell in a liquid crystal display.
Another object of the invention is to provide a liquid crystal display in which the displaying screen is uniformly compensated by an optical compensatory sheet to improve the displaying quality such as viewing angle.
A further object of the invention is to provide an optical compensatory sheet that uniformly compensates a liquid crystal cell without causing troubles of light leaks.
A furthermore object of the invention is to provide a liquid crystal display in which troubles of light leaks are reduced.
The applicant has found that an optical compensatory sheet having an even optical character in the plane can be obtained by stretching a transparent film under specific conditions. The applicant has also found that the troubles of light leaks can be reduced by stretching the transparent film under specific conditions.
The first embodiment of the present invention provides an optical compensatory sheet comprising an optically uniaxial or optically biaxial transparent stretched film, wherein a Re retardation value in plane of the transparent stretched film measured at 550 nm fluctuates, in any direction parallel to the transparent stretched film surface, within the range of xc2x15 nm based on the average Re value in each direction, and a Rth retardation value along the thickness direction of the transparent stretched film fluctuates, in any direction parallel to the transparent stretched film surface, within the range of xc2x110 nm based on the average Rth value in each direction.
The first embodiment of the invention also provides an optical compensatory sheet comprising an optically uniaxial or optically biaxial transparent stretched film and an optically anisotropic layer formed from discotic liquid crystal molecules, wherein a Re retardation value in plane of the transparent stretched film measured at 550 nm fluctuates, in any direction parallel to the transparent stretched film surface, within the range of xc2x15 nm based on the average Re value in each direction, a Rth retardation value along the thickness direction of the transparent stretched film fluctuates, in any direction parallel to the transparent stretched film surface, within the range of tic nm based on the average Rth value in each direction, and the discotic liquid crystal molecules are so aligned that an average inclined angle between the discotic plane of the molecule and the surface of the transparent stretched film is less than 5xc2x0.
The second embodiment of the present invention provides an optical compensatory sheet comprising an optically uniaxial or optically biaxial transparent stretched film, wherein the transparent stretched film has a breaking extension along a stretching direction within the range of 10% to 30%.
The second embodiment of the invention also provides an optical compensatory sheet comprising an optically uniaxial or optically biaxial transparent stretched film and an optically anisotropic layer formed from rod-like liquid crystal molecules, wherein the transparent stretched film has a breaking extension along a stretching direction within the range of 10% to 30%, and the rod-like liquid crystal molecules are so aligned that an average inclined angle between the long axis of the molecule and the surface of the transparent stretched film is less than 5xc2x0.
The invention further provides an ellipsoidal polarizing plate comprising two transparent protective films and a polarizing membrane provided between the transparent protective films, wherein one of the transparent protective films is one of the above-mentioned optical compensatory sheets.
The invention furthermore provides a liquid crystal display comprising two polarizing plates and a liquid crystal cell of VA mode provided between the plates, said polarizing plate comprising two transparent protective films and a polarizing membrane provided between the transparent protective films, wherein at least one of the transparent protective films placed between the liquid crystal cell and the polarizing plates is one of the above-mentioned optical compensatory sheets.
In the present specification, the term xe2x80x9cslow axisxe2x80x9d means the direction giving the maximum refractive index in the plane of the film.
The term xe2x80x9cstretching directionxe2x80x9d means the direction in which the transparent film is stretched. In the case where the film is multi-axially stretched, the term means the direction in which the film is stretched by the largest times. The angle between the slow axis and the stretching axis is so determined that it may be an acute angle, and whether the angle is positive or negative is determined based on the stretching axis.
The Re retardation value in plane and the Rth retardation value along the thickness direction of the transparent film are defined by the following formulas (I) and (II), respectively:
Re=(nxxe2x88x92ny)xc3x97dxe2x80x83xe2x80x83(I) 
Rth=[{(nx+ny)/2}xe2x88x92nz]xc3x97dxe2x80x83xe2x80x83(II) 
in which each of nx and ny is a refractive index in the plane of the film; nz is a refractive index along the thickness direction; and d is the thickness of the film.
The Re retardation value in plane of the optical compensatory sheet is defined by the following formula (I):
Re=(nxxe2x88x92ny)xc3x97dxe2x80x83xe2x80x83(I) 
in which each of nx and ny is a refractive index in the plane of the sheet; and d is the thickness of the sheet.
The retardation value along the thickness direction of the optical compensatory sheet is defined by the following formulas (II):
Rth=[{(nx+ny)/2}xe2x88x92nz]xc3x97dxe2x80x83xe2x80x83(II) 
in which each of nx and ny is a refractive index in the plane of the sheet; nz is a refractive index along the thickness direction of the sheet; and d is the thickness of the sheet.
The optical compensatory sheet or the ellipsoidal polarizing plate may be cut (or punched out) according to the size of a liquid crystal cell to be used with the cell.
In the present specification, the terms xe2x80x9cessentially parallelxe2x80x9d, xe2x80x9cessentially perpendicularxe2x80x9d or the like means that the difference from the exact angle is within the range of xc2x15xc2x0. The difference is preferably within the range of xc2x14xc2x0, more preferably within the range of xc2x13xc2x0, further preferably within the range of xc2x12xc2x0, and most preferably within the range of xc2x11xc2x0.
The transparent film used in the optical compensatory sheet of the first embodiment (comprising a stretched birefringent transparent polymer film, or comprising a transparent film and an optically anisotropic layer formed from discotic liquid crystal molecules) is stretched under the conditions controlled to make the optical characters (retardations in plane Re and along the thickness direction Rth) even in the plane of the sheet. The applicant has succeeded in correcting the uneven optical compensatory character (viewing angle changing according to the viewing position on the displaying screen) of a liquid crystal display. This correction can be realized with a large viewing angle maintained.
Further, an optical compensatory sheet comprising the above transparent film and a thereon-provided optically anisotropic layer in which discotic liquid crystal molecules are aligned under the condition that an average inclined angle between the discotic plane of the molecule and the surface of the transparent film is less than 5xe2x80x2 is prepared. The applicant has also succeeded in optically compensating correctly the whole plane of the liquid crystal cell comprising essentially vertically aligned rod-like liquid crystal molecules.
An optical compensatory sheet is generally cut (or punched out) to use according to the displaying size of the liquid crystal display. Since a conventional compensatory sheet has laterally uneven optical compensatory characters, liquid crystal displays equipped with pieces of the cut (or punched) compensatory sheet have different displaying quality. In contrast, liquid crystal displays equipped with pieces of the compensatory sheet according to the invention have constant displaying quality.
Further, a polarizing plate as well as a liquid crystal cell generally has a viewing angle character. The applicant note that the viewing angle character of polarizing plate can be optically compensated very effectively by an optical uniaxial or biaxial (preferably, biaxial) transparent film having laterally even optical characters.
The applicant further has succeeded in reducing troubles of light leaks by adjusting the breaking extension along a stretching direction of the transparent stretched film within the range of 10% to 30% (the second embodiment of the invention). A liquid crystal cell comprising essentially vertically aligned rod-like liquid crystal molecules can be optically compensated by using a optical compensatory sheet comprising the transparent stretched film (and an optical anisotropic layer comprising rod-like liquid crystal molecules so aligned that an average inclined angle between the long axis of the molecule and the surface of the transparent stretched film is less than 5xc2x0) without causing troubles of light leaks.
The optical compensatory sheet according to the present invention is advantageously used for a liquid crystal display having a liquid crystal cell comprising essentially vertically aligned rod-like liquid crystal molecules (e.g., liquid crystal cell of VA, OCB or HAN mode).